1. Field of the Invention:
The present invention relates to an arc extinction chamber unit in a multipolar circuit breaker.
2. Description of the Prior Art:
Prior art arc extinction chamber units are depicted in FIG. 13 and FIG. 14. An arc extinction chamber unit 100 illustrated in FIG. 13 is designed with grids 101, each formed with a V-shaped groove, sandwiched in between insulator side plates 102. The arrangement of the arc extinction chamber unit shown in FIG. 14 differs in that grids 201, each formed with a U-shaped groove, are held by insulator plates 202 which are bent in a U-shape. The grids 101 and 201, in FIGS. 13 and 14 respectively, are fixed by inserting projections 103 and 203 provided at both ends thereof into substantially rectangular slits formed in the insulator plates 102 and 202, and by caulking them in place.
FIG. 15 illustrates an example of a conventional circuit breaker 300 equipped with a prior art arc extinction chamber unit.
In FIG. 15, an outer insulator box 301 accomodates a circuit breaker mechanism consisting of a movable contact 302, a fixed contact 303, an opening/closing mechanism 304, an operation handle 305 and a tripping device 306. The arc extinction chamber unit 100 is disposed in an area of the breaker mechanism swept out by the movable contact 302.
The circuit breaker 300 breaks an accidental overcurrent, whereby the movable contact 302 is separated from the fixed contact 303. At this time, an electric arc generated between the contacts is attracted to the grids 101 of the arc extinction chamber unit 100 by an electromagnetic force. The generated arc is cut off by the grids 101 and at the same time cooled, thereby extinguishing the arc. The breaking process is thus completed without arc damage to contacts 302 and 303.
In the prior art, the insulator plates 102 which hold the grids 101 serve to evolve a gas for furthering the arc extinction due to thermal decomposition when being exposed to an intense heat of the electric arc, thus accelerating the cool-down of the arc. The gas is instantaneously generated, and hence an instantaneous pressure is exerted on the inner walls of the outer box 301 of the circuit breaker 300. In general, it is a common practice that the gas is then vented to prevent the outer box 301 from being ruptured by over-pressure.
There are, however, the following defects inherent in the prior art arc extinction chamber unit.
(1) When installed in the multipolar circuit breaker, it is necessary to incorporate an arc extinction chamber unit for every pole. This incorporation requires excessive labour. Further, because the grids are sandwiched in between the relatively-easy-to-deform insulator plates, very careful installation is needed, further slowing the assembly process. When the assembly is performed by using an automatic apparatus such as a robot, the arc extinction chamber unit can be easily damaged.
(2) The construction of the arc extinction chamber comprises a combination of grids and insulator plates, resulting in several openings and vents. It is therefore impossible for the arc extinction chamber unit to contain the pressure of the gas instantaneously evolved due to the electric arc. The gas pressure is vented directly to the outer box of the circuit breaker. Hence, the outer box is required to have a strength sufficient to endure the gas pressure. As a breaking capacity of the circuit breaker increases, the energy necessary for the breaking process augments. This results in an increment in the amount of generated gas, and the strength of the outer box must withstand this additional pressure. For this reason, the outer box must be built very thick, with the result that the box becomes physically large. Further, more material is required for construction of the box. The result of this is that the physical structure of the circuit breaker box may limit the breaking capacity of the circuit breaker.
(3) After a plurality of grids have been interposed between the insulator plates, it is necessary to caulk them in place. This process is complicated and increases the number of required assembly steps. The process is therefore unsuitable for mass production.
(4) Since the insulator plates are exposed to the intense heat associated with the electric arc, the insulator plates become burnt and damaged. This results in the grids falling away from the insulator plates. In order to prevent this degradation, a reinforcement is required. For example, a heat-resistant adhesive-backed tape can be applied over the caulked sections. This step however increases the overall cost.